(1) Field of the Invention
The present invention relates to a process for the production of a covered deep-drawn can. More particularly, the present invention relates to a process for the production of a covered deep-drawn can which is excellent in the adhesion of the covering and the corrosion resistance, heat resistance and denting resistance.
(2) Description of the Related Art
As the process for the preparation of a side-seamless can, there is known a process comprising subjecting a metal blank such as an aluminum sheet, a tin-plate sheet or a tin-free steel sheet to a drawing operation of at least one stage between a drawing die and a punch to form a cup comprising a barrel having no seam on the side and a bottom integrally connected seamlessly to the barrel, and if desired, subjecting the barrel of the cup to an ironing processing between an ironing punch and an ironing die to reduce the thickness of the barrel. It also is known that a laminate formed by laminating a film of a thermoplastic resin such as polypropylene or a thermoplastic polyester is used in the production of this side-seamless can. Japanese Examinated Patent Publication No. 59-35344 and Japanese Examined Patent No. 61-22626 teach that in order to moderate the internal stress generated in a covering resin layer at the drawing or deep-drawing (redrawing) step, the formed vessel is heated at a temperature close to the melting point of the resin and is then cooled.
According to this conventional technique, the molecular orientation imposed on the resin film layer at the drawing or deep-drawing processing of the laminate blank is moderated and the resin is rendered amorphous, whereby the adhesion of the resin layer to the metal substrate is improved. However, this method is defective in that the corrosion resistance or heat resistance of the obtained can body is still insufficient.
It is known that in a thermoplastic resin film, the barrier property to a corrosive component is higher as the molecular orientation or crystallization degree of the resin is higher, and that also the mechanical properties such as the strength and impact resistance are higher as the molecular orientation degree of the resin is higher. Accordingly, if the molecular orientation is moderated and the resin is rendered amorphous as proposed in the above-mentioned conventional technique, this results in degradation of these characteristics of the molecularly oriented resin.
Furthermore, in case of a crystalline thermoplastic resin such as polyethylene terephthalate, bad influences are brought about by the heat crystallization. For example, in a resin covering as mentioned above, heat crystallization (spherulitization) is caused at a cansterilization temperature, and the characteristics of the covering are drastically degraded.